Knowledge Base

The picture above shows some of the bidirectional digital I/O pins used as outputs.

Each 6-pin digital I/O connector has a pin, labeled “+V”, connected to power supply positive. The output load should always be connected between the output pin and power supply positive. Use the “+V” pin in preference to wiring directly to the power supply +.

CAUTION: Take care never to short an output pin to the positive common pin. If the output is on at the time you will blow up the output device instantly and possible cause irreparable damage to the board.

The loads illustrated are:

A: The input pins of a solid state relay. Note the polarity! Connect the negative terminal of the device to the HMI430 output.

B: A relay coil. Notice we have included a catch diode across the coil. While there are diodes on the board we recommend a diode across the coil, as close as possible to the coil, to minimize problems with switching noise. A suitable diode is a 1N4001 or similar 1A/50V rectifier diode.

C: An LED, with a series current limiting resistor. Resistance calculators

There are catch diodes built into the chip. This means the board will not be damaged if you are driving relay coils, solenoid valves or other inductive loads (within the ratings of the output). We nevertheless recommend external catch diodes connected directly to the relay coil as close as possible to the relay, in order to reduce adverse effects of switching noise. The on board diodes also mean that the load must not be connected to a supply voltage greater than the board’s positive supply voltage.

If you connect the loads (for example relays) separately to the power supply or to a separate power supply, it may be possible to remove the controller’s supply connection and still have power fed to the controller via the loads and the on board protection diodes. This can produce confusing or possibly even dangerous results if the controller gets enough of this “sneak” power to run. It is safer to use the “+V” supply pin on the I/O connector to provide positive voltage to the load.

There is a maximum allowable current per output, plus a maximum aggregate current (the total of all outputs). Please see the specification page for the actual numbers.